M20 2.7 stroker idle troubleshooting

[ramerco]

I've tried a good about of searching and reading and finally time to post my specifics.

I've recently built a 2.7 stroker of the following:

Bottom end:
81mm crank
130mm rods
325i pistons
Block is decked to bring piston height back close to stock M20B25 level (i think it is 5 or 10 thou short of normal 325i, so the compression may be 9.15 or something instead of planned 9.3)
Stock head gasket

Head:
Refreshed but fully stock 325i (885) head)
Adjustable cam gear

Intake side:
M30 Vane AFM
Overbored TB

Other:
SSQuid chip specific for this build
LC-2 with AFR gauge
Oil pressure and temp gauge


First off, the car runs well. On first start I got what felt like a lot of exhaust smoke, so I cut it off after about 2 minutes. I checked things over and was happy and then ran break-in of 20 minutes between 1500 and 2000 rpm then drove it. I've put about 75 miles on it and its going well at load and has plenty of power throughout the band. Haven't taken it above 5k rpm yet.

The issue is at idle. Initial cold idle is fine, but a little high ~1000rpm, but I believe that is normal. I can adjust the VAM to get AFR readings anywhere from 12 to 15 and it is smooth at 1000rpm and fairly steady on AFR, +/- 0.5. When it warms up a touch (2 mins?), the idle begins to have some trouble. It will go lean periodically for just a half to one second, jumping from 14.5 range to 16+, and the engine shakes a bit as the RPM stutters. The shake is noticeable in the cabin and feels like it is drooping in RPM (although the gauge doesn't always indicate this because it is quick). This really make me think vacuum, but I've really tried to rule that out.

Compression is ~150 across all pistons, but I feel like this is low because the same gauge showed 175 psi on the stock moter (but that was during hot days of summer). I have run vacuum gauge and on idle it drifts down to a little under 15" Hg, also a little low. It is well into 20-25 range when revving, coming off throttle it drops below 20 then settles out at steady 14-15.

I have looked extensively for vacuum, leaks. Including a smoke test. I can't find anything. If i pull something I can get smoke to come through, but when all tight I find no leaks. When running I can pull the FPR hose or oil cap and it does make the lumpy idle worse, but not a huge change. The ICV checks well (piston rotates and i get the required 20 and 40 ohms). The TPS also checks out. VAM voltage is steady and the arm doesnt move when this occurs.

I have an adjustable cam gear and have advanced the timing a bit (about 5 deg) and the compression came up a little (all at or above 150 from I think 140ish - i didn't record the compression before the adjustment). I am wondering if my initial timing was off by a tooth (the TDC mark on my cam gear was not clear), so maybe I need to advance more. But I am also concerned about contact.

If timing is off by a tooth, I still should not get any contact, right? If I continue to advance timing will my compression drop before I get so far that I would get valve-piston contact?

Could the idle issue be related to potentially retarded timing? Any other ideas? A miss would cause it to go rich, right? Going briefly lean makes me feel like it is an air related problem.

[ramerco]

I have also done the Fuel filter and FPR just as maintenance items.

Any other tips how to search for vacuum leaks? I did try pull the brake booster and the engine stalls. Some reading suggests it should keep running but rough with that disconnected.

I may just replace all the injector o-rings to rule that out.

[ramerco]

Bumping my own thread. Anyone have any suggestions for an idle that hunts a bit? Its just a small surge, like 50 rpm change. Also tried searching for idle dip, pulse, etc. Seems like most issues are a lot bigger. This will go 2-3 sec and the stutter. AFR goes LEAN when it stutters, so to me (assuming I really don't have a mystery vac leak) that points at injector changing pulse width, like the ECU is asking for less fuel. At this point I have dialed in the mixture so the car doesn't get lean enough to shake much (the 'normal' idle is ~13 and then it drops to 15-16 when it stutters). Maybe I will take the wideband out and try with a stock narrow band sensor.

[moatilliatta]

If timing is off by a tooth, I still should not get any contact, right? If I continue to advance timing will my compression drop before I get so far that I would get valve-piston contact?


It'll still run with timing being off a tooth. Cam timing being off a tooth cold show 150 PSI in the compression test.

You can also spray starting fluid by the suspected intake leaks to check them that way and look for a change in idle. Some intake leaks wont show under pressure of smoke but will under vacuum.

[ramerco]

Update here. Ended up being primarily the ICV wiring harness. I cleaned the contacts on the wiring harness side of the connector and it seems 95% fixed.

I do think my timing was a bit retarded and I could have a little air/vac leaks still. But the hunting is basically gone now. Vacuum test shows a little fluctuation (+/- 1”) at idle and a little low 17inhg instead of 20.